It is known that reinforcement materials that are placed with a small angle relative to the equatorial plane (wound spirally) on the belt package especially improve the high speed performance in radial tires. Using polymeric (nylon 6.6, PET, aramide/nylon hybrid etc.) cords as cap ply reinforcement by being wound on the belt package spirally as strips has been applied for many years by several companies in order to improve high speed durability and handling performance in pneumatic radial tires. The cord strips are obtained by cutting calendered (rubberized) cord fabric in strips or rubberizing parallel single cords in a certain width during extrusion process.
The purpose of using the cap ply reinforcement is to increase the high speed durability of the tire by avoiding belt layer separation caused by the centrifugal force occurring in the belt package at high speeds. The intensity of the resistance shown by the tire against belt edge separations is highly important, for high speed durability.
When nylon is used as cap ply reinforcement material, there are two important disadvantages: first, the obligation to use more than one layer due to its low modulus, second, the temporary geometric deformation (temporary flatspot) caused as a result of cooling the tire heated at high speeds upon parking, since the glass transition temperature of the polymer is low.
U.S. Pat. No. 7,584,774, an application known in the state of the art, discloses the use of high modulus polyethylene terephthalate cord as cap ply reinforcement material in order to increase the high speed durability of a tire and decrease the temporary geometric deformation (flatspot) problem. In this embodiment the tangent modulus of the cap ply cord is suggested being higher than 2.5 mN/dtex. % at 16° C. under 29.4 N. For this reason, it is suggested that the process expansion will be a maximum of 2% to avoid tight cord formation which can cut through the rubber coating of the belt.
In order to increase the high speed durability of the tire, the cap ply reinforcement cords located on the belt package should resist the tire growth that can occur from the centrifugal force at high speeds.
The said tire growth resistance formed in the cap ply reinforcement cords is comprised the total of
A. force/stress (cold residual tension) occurring in the cap ply reinforcement cords after vulcanization process,
B. thermal shrink force generated doe to increasing temperature of the belt zone because of high speed,
C. and the cord modulus at the said temperature.